Commercial HVAC units including a heating function in addition to a cooling or cooling/dehumidifying function contained in a common cabinet are commonly used for heating, ventilating and air conditioning on commercial buildings. Typically, such packaged HVAC units are mounted on the rooftop of the building and connected into an air duct system associated with the building for passing air from within the building to be temperature conditioned to the HVAC unit and distributing temperature conditioned air throughout the building. In some applications, packaged HVAC units are ground mounted outside the building, rather than being mounted on the rooftop of the building.
Such year-around packaged HVAC units include an indoor section and an outdoor section housed within a common cabinet, but segregated by partitions and walls into separate compartments. The outdoor section includes one or more condensing units, each having a condenser heat exchanger coil and an associated fan, as well as other components such as a refrigerant compressor. The indoor section includes an air conditioning module, having an evaporator heat exchanger coil and an associated evaporator fan or fans, and an air heating module.
Commonly, the air heating module includes a furnace heat exchanger and an associated combustion system. In one type of air heating module, the furnace heat exchanger comprises a plurality of elongated hairpin-configured tubes defining an interior flow passage through hot flue gas generated by the associated combustion system is passed. The furnace heat exchanger is disposed in an air heating plenum through which indoor air is passed over the external surface of the heat exchanger tubes in heat exchange relationship with the hot flue gas passing through the furnace heat exchanger tubes.
The air heating plenum is typically disposed downstream with respect to air flow of the air discharge outlet of one or more indoor air fans. The heat exchanger tubes extend horizontally across the air heating plenum. The flue gas passing through each of the heat exchange tubes is hottest at the inlet end of each hairpin-configured tube and coolest at the outlet end of each hairpin-configured tube. In some designs, the air passes vertically downwardly over the heat exchange tubes to discharge downwardly through an opening in the floor of the air plenum. In other designs, the air passes vertically downwardly into the air heating plenum, but turns through 90-degrees within the plenum to exit horizontally through an opening in an end side of the air heating plenum. Thus, the air flow passes both across and along the hairpin-configured tubes of the furnace heat exchanger thereby increasing the time for which the air flow passes in heat exchange relationship with the flue gas relative to applications in which the air flows directly vertically downwardly over the heat exchange tubes.
The effectiveness of the heat exchange will be adversely impacted, that is heat exchange from the flue gas to the air will be reduced, in regions that may have relatively low air flow across or along the heat exchange tubes. The reduction of air flow across or along the heat exchange tubes may occur in certain regions due to the turning of the air flow as it passes through the air plenum. The reduced heat transfer may result in high temperature zones, i.e. hot spots, of the surface of the heat exchange tubes, which can also adversely impact reliability of the heat exchanger. Common steps taken in an attempt to provide a more uniform air flow across the heat exchanger and/or increase air flow in regions experiencing weak air flow, such as increasing the fan exhaust and air discharge opening areas, installing large fans, adding flow baffles within the heat exchanger, or adding fins on the heat exchanger tubes, add increased cost to the system and are not always effective in improving heat transfer to the air flow.